Investigating relationships between climate controls and nutrient flux in surface waters, sediments, and subsurface pathways in an agricultural clay catchment of the Great Lakes Basin
Nutrient balances in headwater catchments of the Great Lakes Basin are critical to the long-term health and productivity of the Great Lakes. Nutrient export within an agricultural catchment is governed by management practices and climate conditions that control the transport, storage, and exchange of nutrients between hydrologic compartments. A 14-month field study was completed in a lowland clay hydrosystem headwater catchment of the Lake Huron Basin. Monthly and event-based discrete samples were collected from surface water, groundwaters, tile drain discharge, sediments, and from the hyporheic zone to determine phosphorus (P) and nitrate (N) concentrations. Evidence suggests tile drains flow seasonally and are a source of soluble P and N to surface waters and can convey nutrients from both diffuse sources and legacy stores from the unsaturated zone. Hyporheic exchange and streambed sediments may offer nutrient retention within the stream, but currently have no discernable benefit to year-round surface water quality. Moderate (< 2-year) rainfall-runoff events following fertilization resulted in elevated concentrations of P and N throughout the catchment and deviated from the classical high-risk times of nutrient loading. Sediment transport modeling revealed events of varying magnitude were found to mobilize fines sediments (clays, very fine sands, fine sands) within the stream course, resulting in erosion, deposition, and remobilization dynamics. Best management practices need to focus on reducing sources of diffuse agricultural nutrients (e.g., field phosphorus concentrations) and point sources (i.e., tile drain discharge) at the watershed scale to reduce nutrient concentrations and export in flashy clay catchments.